First Council of Nicaea: first of the seven Ecumenical Councils in which Christian doctrine was established (325).
- Organized by the emperor Constantine I the Great;
- recognized by pope Sylvester I;
- discussion of the teachings of Arius, who maintained that Christ was created by God;
- dogma that Christ is fully God and equal to the Father;
- the Nicene Creed established;
- rules for the computation of the Easter date.